A mobile telecommunications system such as a cellular mobile telephone (CMT) or personal communication services (PCS) system, a geographic service area is divided into regions, or unit "cells". Each cell is equipped with a central radio transceiver, known as a base transceiver system (BTS) that employs a predetermined set of radio frequencies. The same radio frequencies are then repeated several times across the service area as long as the cells using them are not neighbors. As a radio telephone, or mobile station (MS), moves through this pattern of cells, telephone calls--made from the user's perspective in much the same manner as on regular telephones--are switched from one cell to the next by a computerized system known as a mobile switching center (MSC), sometimes also known as a base station controller (BSC).
In the typical system architecture a mobile switching center manages the connections to a public switched telephone network (PSTN) for a number of BTSs. The BTSs are responsible for generating the appropriately modulated and encoded radio signals to remain in radio contact with the mobile stations located within the respective cell. Each BTS is also required to maintain a landline connection with is associated MSC using using one or more suitable landline interfaces. The interfaces must not only carry the voice signals between the BTS and BSC, but must also support control messaging between the BTS and MSC.
Control messaging includes both the messaging which is needed in any telephone switching system to originate and terminate telephone calls, known as call processing, as well as the messaging needed to permit a call to remain in progress as the mobile station moves from cell to cell, known as mobility messaging, during which time the control of the call is "handed off" from one BTS to another.
One increasingly popular way to interconnect the MSC and BTSs is by the use of standardized transport signaling such as integrated services digital network (ISDN) signaling. A common type of ISDN link provides 24 channels on a single physical carrier medium, with the channels allocated as "23B+D", meaning that 23 of the channels serve as so-called bearer (B) channels which carry voice traffic, and a single control or data (D) channel that carries the control messaging.
A number of problems in mobile system design are driven by the ever increasing need for mobile communication services, which typically requires system operators to serve an geometrically increasing number of users in a given service area. For example, even in the well established industrialized nations of the world, the number of mobile telephone users continues to increase at rates of 50% per year and more. As a result, certain types of high capacity base transceiver system (BTS) equipment making use of broadband radio technology have been developed which are intended to service a relatively large number of active mobile stations in each cell. Such broadband equipment can service, for example, ninety-six simultaneously active voice and control radio channels within a single four-foot tall rack of electronic equipment.
Unfortunately, the deployment of broadband radio equipment in and of itself does not solve all of the problems presented by increased user demand. As the number of active mobile stations in a given service area increases the demand on the BTS and MSC resources which are responsible for supporting mobility messaging also increases.
In addition, there are typically multiple handoff attempts from an originating or serving cell to a destination or neighboring cell as a mobile station moves about the service area. Such handoff attempts not only occur as a mobile station moves near the edge of the serving cell, but may also occur as the mobile station move behind buildings and other obstacles produce a "shadowing" or multipath effect in the propagation of radio signals in the serving cell.
While certain newer forms of mobile communication systems known as personal communications systems (PCS) provide for a greater number of users in a given geographic area by deploying a greater number of BTSs in physically smaller cells, the handoff messaging situation is only exacerbated by the prospect of having to handle a greater number of mobile stations located among a larger number of physically smaller cells. The end result is that the service provider is typically left with no choice but to incur the expense of deploying a greater number of MSCs in a given service area.